In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. Energy is extracted from the combustion gases by passing the gases over turbine rotor blades that in turn power the compressor, and an upstream fan in an exemplary turbofan aircraft engine application.
Each rotor blade includes an airfoil extending radially outwardly from an inner platform, with the platform being joined by a shank to a supporting dovetail mounted in a corresponding slot in the perimeter in a supporting rotor disk. During operation, the blades drive the rotor at substantial speed and are subject to centrifugal forces or loads that pull the blades radially outwardly in their supporting slots in the perimeter of the rotor disk. The dovetail typically includes multiple lobes or tangs that carry the centrifugal loads of each blade into the rotor disk while limiting the stresses in the blade for ensuring long blade life.
Each rotor blade is subject to pressure, thermal loads and stresses from the combustion gases that flow over the blades during operation. The blades are also subject to vibratory stress due to the dynamic excitation thereof by the rotating blades and the pressure forces from the combustion gases. The blades are relatively thin to minimize weight and the resultant centrifugal loads, making the blades susceptible to vibratory excitation in various modes. For example, the airfoil may be subject to vibratory bending along the radial or longitudinal span thereof, as well as higher order bending modes along the axial chord direction.
Accordingly, turbine blades may include a vibration damper mounted under the blade platforms. The dampers are supported by the platform and dovetail and add centrifugal loads to the rotor disk. The dampers use friction with the excited platform to provide effective damping of the blade during operation at speed. However, these dampers have limited effectiveness for the various modes of vibration of the turbine blade during operation, including the higher order natural modes of airfoil vibration that involve complex combinations of airfoil bending in both the chord and span directions.
One approach to dampen vibration occurring in the airfoil has been to position dampers within the airfoil of the turbine blade. One approach includes a bipedal damper that includes a pair of wires or pins extending into the flow channels. However, the geometry of these dampers require complex forming processes that are expensive and do not provide for different material characteristics in different positions in the damper. For example, one may require a material with excellent wear resistance in one location where the material of the damper is in contact with the material of the component being dampened, yet also require a material of high strength in another location where the damper is subjected to the same high centrifugal loading seen by the rotor and attached turbine blade. In this case, a cast monolithic damper may be used but may provide less than optimum performance due to defects that can be introduced during the forming operation, sub-optimum wear characteristics that may cause wire failure due to frictional wear, or may rupture due to high tensile loading.
Another known damper design has taken the form of a wire or small diameter bar, measuring about 0.020 inches to about 0.200 inches in diameter and from about 2 inches to about 5 inches in length, that are inserted into a cavity of the turbine blade. These dampers are referred to as wire or stick dampers. The wire dampers are positioned within the airfoil and typically extend the length of the turbine blade. The dampers are in contact with supporting lands formed on the internal wall of the turbine blade. Frictional vibration between the damper and the airfoil dissipates excitation forces and effectively dampens blade vibration.
However, frictional dampening is subject to wear between the damper and the airfoil, and the damper is subject to substantial centrifugal loads during operation and experiences corresponding tensile stresses and bending stresses along its length.
In order to increase blade life, the damper should be formed of a material having sufficient high strength for affecting long low cycle fatigue life, long high cycle fatigue life, and long rupture life. These life factors are typically controlled by the highest steady state stress portions of the damper, which are typically in the supporting portion of the damper in the dovetail.
In contrast, the outer portion of the damper is subject to frictional vibration with the airfoil and experiences lower stresses during operation, but is subject to high frictional wear. Up to this time, blade vibration damper designs fail to strike a compromise between wear and strength performance of the damper.
Therefore, what is needed is a wire damper that provides dampening, is simple to produce, and is simple to include in the blade design. The wire damper should also provide improved wear resistance in combination with high strength.